A Crack in Time
by waffleman1314-AJ ThaPlatypus
Summary: When Candace uses Phineas and Ferb's time machine to travel back to the day that they built the roller coaster, she wasn't planning to affect the space-time continuum in the process. But coming face-to-face with herself spins disaster into motion, leaving the fate of the universe in the capable hands of a time-traveling man and his faithful companion. *Stopped*
1. What Went Wrong

**Here we are! -AJ **

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**Okay, so this is a collaborative crossover with Leonardo Oliver Osborn. :) I've never done Whovian fanfiction before, but hey; something new is never bad :). Tell me what you think of our collab! -AJ ThaPlatypus**

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The day had been going fairly well. If one considered building a roller coaster throughout the Tri-State Area well, that is. The sun was shining, most everyone was happy, and a positive summer outlook was on the air. In all reality, the day would have continued to go on as such a fine day if it had been left alone.

An indeterminable amount of days later, perhaps even several weeks, the same two boys that built the roller coaster created a crude time machine. They thought it'd be the only use of it after an accidental disaster trip to prehistoric times. It wasn't.

Back to the day first mentioned, the same time machine that had yet to be built arrived in a museum. A woman roughly in her early thirties was its sole passenger. She jumped out and dashed out of the museum. She remembered the day; she'd been fifteen at the time. No one questioned as she ran through the streets with an air of determination. It was only a matter of minutes before she located the supermarket that she and her mother were in.

A funny thing, paradoxes are. They cover such a broad scope of things, such as geniuses being incapable of intelligence or the correct way of doing something being at the same time the wrong way of doing something. Depending on the paradox, some are easily fixed with logic, while others are simply gateways to anomalies. In truth, this situation leads to a paradox of a difficult sort: a time paradox.

The woman who travelled back in time was headed straight for the place she had been at that exact time on that exact day. Running into her past self would have a terribly undesirable consequence. Humans never seemed to understand that on their own. Playing with time was as risky as trying to defuse a bomb without knowing which wires to cut. Granted, a more apt comparison might just as easily be enjoying a game of catch with a grenade. This woman was walking on dangerous ground, and it could simply blow up in her face.

Time is a delicate thing. It is pieced together by a series of events. If one person never travels through time, they never see the effects of time in any way other than a straight line. Yet things from the future have had impacts on the past, and vice versa. Traveling through time can both set things in motion that were meant to be and it can change things that were never set in stone. Few can see events in such a manner; even fewer actually care.

The woman burst into the supermarket with purpose. It was too close for comfort, but it was too late as of that moment. Her younger self was gaping at an empty post with incredulity. She smiled a little and looked at her mother, who was standing behind her younger self. The damage had been done. Before she could say what was on her mind, a shriek sounded outside.

Two ran into the parking lot in front of the supermarket and came to a quick stop. They looked up into the sky and saw what appeared to be a crack ripping through it. It didn't matter what the woman they'd seen was planning to do; if they didn't do something, it wouldn't matter anyways.

"Is this what we came here for?" the female of the pair panted, her eyes fixed on the anomaly sitting up in the sky. The man next to her was silent, his eyes also locked on the crack. "Is something wrong?"

"This wasn't supposed to happen," he murmured. He looked down towards the doors of the supermarket to see both selves of the woman walk out of them. Two young boys ran over to them, exclaiming something and pointing up at the sky with wild expressions.

"What do we do?" the female persisted. Again, there was no immediate response. "Are you listening to me? Doctor, what's happened?"

"A paradox," he said grimly.

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**So? First collaborative goes...how?**


	2. Opposing Timelines

**Got a second chapter (finally!) and I hope it's doing alright, hehe. -AJ **

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As he approached the boys and the women, the Doctor had already set about trying to solve the issue at hand. A woman had traveled to her own past and run into her younger self in a way that was seemingly intentional. That meant that she was either a first-time time traveler or she was clearly not aware that crossing her own timeline would be dangerous. He shoved his hands into his pockets and walked on, keeping his eye fixed on the group. From the look of it, they were completely confused by the anomaly taking place above them.

"Doctor, who are they?" his companion asked, nodding in the direction of those they were approaching. Her question was quickly followed by another. "Anyone famous?"

"Clearly child prodigies," the Doctor glanced up at the roller coaster. He would have been more impressed with the young humans had he not been focused on the paradox at that moment. "The ginger girl, though, is the one we're concerned with, Martha. She's some kind of amateur time-traveler, from what I gather."

"Ah," Martha replied, unsure of what to respond with. The Doctor picked up his pace a little and waved one hand at the group. All four turned to face him with a confused expression plastered across each of their faces. At least they were staying put instead of running off.

"Hello!" the Doctor smiled a little. He pointed up at the sky. Their eyes followed his gesture. "It would seem that there's a bit of a paradox problem here, yes?"

The group remained silent. They continued to give him their blank expressions. No one present was aware of the situation in its entirety. Pulling out his sonic screwdriver, the Doctor walked over to the older version of the woman and looked her up and down. She edged back a little.

"You're from a different time, yes?" the Doctor asked, not really expecting an answer. She swallowed. He waved the screwdriver in front of her. She jumped at the buzzing sound a little and gave him a slight frown. "Ah, yes you are. Tell me, Miss, how exactly did you come back to this time?"

"I…uh…used the time machine that my brothers built," she stammered, pointing at the two boys. They straightened up at their mention and stopped staring at the sky. "And my name's Candace. Candace Johnson."

"I _do_ marry Jeremy!" the younger girl shrieked. The older one almost smiled. The Doctor ignored them and turned to the two boys.

"You boys built a time machine?" he lifted an eyebrow. Both shook their heads. The denial wasn't looking to place things in the Doctor's favor. He sighed.

"No," the redhead with the triangle-shaped head said, "but that does sound fun! Put that on the list, Ferb!"

The other boy said nothing, but he pulled out a notepad and started to scribble something down onto it. Humming, the Doctor paced in a circle a few times and then turned to Martha with thought. His companion lifted her head a little and waited for his explanation.

"So, the older Candace traveled to this time with a time machine built by her brothers in their future from this current point in time," he summarized quickly. Running his fingers through his hair, he looked around the area again. "See what I mean about time, Martha Jones? Everything is all jumbled up. The past is affected here by two different futures- one that should happen, and one that shouldn't exist."

"Is there an evident fix?" Martha frowned. "Tell me there's an evident fix."

"Well, we don't have much time before the paradox starts to eat away at time itself," the Doctor hummed. He whirled around to the older Candace and pointed at her. She took a step back. "You; you said you arrived here by a time machine your brothers built. Where exactly is this time machine?"

"Why?" she crossed her arms. "Why should I tell you?"

"Because I can fix this," he told her, pointing up at the crack. Her eyes flitted from the anomaly and then back to him. She didn't say a word. "Look, the world as you know it is about to cease to exist. I'm the only help you've got, Candace Johnson."

The woman showed no signs of considering his statement, but she didn't move to deny him either. Her stubbornness plucked at the Doctor's nerves. He held his stare. She held hers. There wasn't much time to lose. If he didn't win over her trust with the short moment he'd been given, the paradox would do its damage.

"Follow me," she said, turning on her heel.

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**Okay, it's taken forever, but here it is!**


	3. Amateurs

**Here we are! -AJ **

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Candace Johnson led the group without words. While her younger persona was chatty, suspicious, and overall annoying, the Doctor noted her older self was quite the opposite. He attributed it mentally to maturity. Something in her life had changed, and it was the only option. Yet he feared that something outside of her adult mindset had been the cause. After all, she seemed pretty level-headed in general after agreeing to show him the time machine.

Martha kept close at his side and the kids followed a few paces behind. It wasn't that they were wary of him; no, only the younger Candace gave off the vibe that she found him unsettling. Rather, the two boys present were more so intrigued by him. They'd had similar looks of intense interest ever since they'd seen him pull out the sonic. Technology caught their eye and a stranger wasn't about to shun them.

The elder Candace halted them in an open field and pointed in front of herself. What looked to be a chair with red fabric cushions attached to a metal boat sat by its lonesome in the middle of the clearing. She placed her hands on her hips and frowned at the piece with irritated familiarity.

"That's it," she sighed. "That's future Phineas and Ferb's time machine."

Phineas. So that was the redhead boy's name. The Doctor glanced at the boys before approaching the creation. It had clearly been made from already used parts; he wasn't too concerned with the making, though. He was concerned with the fixing. He pulled the sonic screwdriver back out and waved it slowly over different parts of the machine.

"When was this first built?" he questioned, to no one in particular. Candace Johnson gave a reply nonetheless.

"In a few weeks from the time we're in now," she said, casting a glance at her brothers. They smiled, probably anticipating their invention. The Doctor would have normally warned them about learning of their futures, but he suspected they wouldn't remember a thing once the paradox was righted.

"And when exactly did you come from, Mrs. Johnson?" the Doctor frowned. So the time machine was built after this date. That made things a little easier. "How many years from now did you return from?"

"Twenty years into the future today," she said, crossing her arms.

"Motivation for doing so?" he hummed, ducking down and looking beneath the machine. She waited for the buzzing of the sonic to stop before answering.

"My brothers time traveled to that particular day in order to retrieve some tool," she explained. The Doctor did his best not to groan. Things were starting to look like an ill-guided chain of events. "My old self took over and wanted to bust them for doing dangerous things like time travel at their age. I had no idea that all _this_ would happen."

"This happened," the Doctor filled her in, reappearing from under the boys' time machine and standing once again, "because you crossed your own timeline and came face to face with yourself. Your past self wasn't expecting to see your future self, thus it created a paradox. Time tried to rewrite itself so that it would work, but it can't because it wasn't supposed to happen. This is what happens when amateurs time travel!"

"Doctor," Martha said, her tone sounding slightly scolding. He took a deep breath and turned back to the machine. The fix was simple, but it would take a lot of convincing. He'd have to travel back to the date when the two boys decided to go and visit the future. In order to find that out, he'd have to travel to the day that Candace Johnson had just left. He faced the group and slipped the sonic back into his pocket.

"Right, the solution is simple in theory, but it's going to be a little difficult to solve," he told them. He ran his fingers through his hair. Judging by the looks on the boys' faces, he'd have trouble convincing the boys not to have the desire to come with him to fix this. "I need to try and travel back to the date that Mrs. Johnson first left."

"Can we technically do that in the middle of a paradox?" Martha frowned.

"We can try," he sighed. "The paradox being present, we will most likely still see the crack…not to mention the driving trouble we may experience-"

"You have your own time machine?" Phineas piped up. "Can we see it?"

"Look, it's difficult enough to fix this without you tagging along," the Doctor looked down at the boy. The redhead wasn't bothered by the statement in the least. It was good until the Doctor realized the boy had no intention of giving up the pursuit.

"We're really handy with tools," he continued. "If you say you'll have driving trouble, let us help. There isn't a machine in the world we couldn't tackle. Obviously we have the brains to build a time machine of our own. Assisting yours should be a piece of cake."

"I don't think you understand what you're implying," the Doctor furrowed his brow a little. He couldn't imagine anyone, even these two brilliant boys, being able to properly work on his ship. As much as he admired their will to help and their self-confidence, he didn't really find it feasible.

"I think I do," Phineas smiled. He wrapped one arm around the shoulders of his brother and pointed between them. "We make a pretty valuable team. Face it, sir, you might end up needing us to fix this paradox."

"It may cause more problems, as I need to cross your future paths," the Doctor said, trying his best to be firm with them. They were pretty persuasive. But they still didn't have the mind to help in the way they were thinking.

"And if _they're _going," the younger Candace growled. "_I'm_ going. I won't let these two out of my sight."

"Yeah, Candace is pretty useful, too!" Phineas insisted. "Give us a chance—if we can't help out on your time machine, then we'll accept that. But you've got to at least let us try. You said yourself problems could arise in trying to travel in it during a major paradox. Having extra hands that can help the machine isn't a bad idea. It's actually safer."

"I don't know…" the Doctor hummed.

"Perhaps you should show them why," Martha suggested. "They can make their decision once they've seen her."

"That's a fair decision," Phineas nodded. "Lead on to your machine!"

"Alright," the Doctor gave in. "But you haven't the slightest idea what you're in for."

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**I recently vacationed in Florida, and my parents decided to stop at a little shop that sold stone statues. My sister and I glanced at one another, swallowed hard, and then both exclaimed "DERN'T BLINK!"**


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